Meeting Abstract
58.6 Jan. 6 The influence of photoperiod on pulsatile urea excretion in the gulf toadfish Opsanus beta WALSH, PJ*; BARIMO, JF; MCDONALD, MD; University of Ottawa; Portland State University; University of Miami pwalsh@uottawa.ca
The gulf toadfish Opsanus beta (Batrachoididae) is one of the few teleosts to maintain a functional ornithine-urea cycle (O-UC) during adult life and to possess the capability to change from ammonotely to ureotely within 24 hours in the laboratory. Captive O. beta excrete most nitrogenous waste across the gill membrane. In the lab, urea is generally excreted in daily pulses of 1.5 hrs in duration while ammonia is eliminated continually. At present, the mechanism of O-UC activation is putatively due to an elevation of plasma cortisol (stress response) which promotes the up regulation of the key O-UC enzyme glutamine synthetase. Experiments in this study examine the diel pattern of nitrogen excretion in the laboratory and in mesocosms. Under both experimental conditions toadfish were exposed to natural photoperiod, samples were collected hourly from toadfish in shelters fabricated with PVC pipe, and assayed for urea and ammonia with standard chemical techniques. In laboratory trials, conducted in 2 L containers with static seawater changed daily, urea pulses occurred at random with no correlation to light or dark cycles. In mesocosm experiments, toadfish were unrestrained in 8000 L tanks with the seagrass Thalassia testudinum planted on carbonate substrate effectively simulating their natural habitat. Shelters were outfitted with an underwater IR camera connected to a time-lapse video recorder to document toadfish behavior. As in prior field studies, in mesocosms urea and ammonia were excreted simultaneously in approximately a 50:50 ratio and occurred predominately during daylight hours with peak levels near dawn or dusk, and these excretion patterns were also reflected in appropriate and pronounced declines in plasma levels of urea. Differing results between experimental regimes are believed to reflect the degree of stress encountered by toadfish and to represent two ends of the spectrum of natural physiological responses. The results will be discussed in the context of a hypothesis of chemical crypsis and predator avoidance. Supported by NSF (IOB-0455904)
